The present invention is related to hearing systems and, more particularly, to implantable microphone devices that may be utilized in hearing systems.
Conventional hearing aids are placed in the ear canal. However, these external devices have many inherent problems including the blockage of the normal avenue for hearing, discomfort because of the tight seal required to reduce the squeal from acoustic feedback and the all-too-common reluctance for hearing-impaired persons to wear a device that is visible.
Recent advances in miniaturization have resulted in the development of hearing aids that can be placed deeper in the ear canal such that they are almost unnoticeable. However, smaller hearing aids inherently have problems, which include troublesome handling and more difficult care.
Implantable hearing devices offer the hope of eliminating problems associated with conventional hearing aids. One requirement for a fully implantable hearing device or system is an implantable microphone.
All microphones necessarily contain an interface between the internal components and the environment in which it will be situated. For non-piezoelectric designs, air-conduction microphones utilize a membrane, which can be made of various materials, stretched or formed to varying tensions. The tension in the membrane has a first order effect on the response of the microphone. A highly stretched membrane will tend to resonate at a high frequency, with a flat response at frequencies below the resonance. However, a higher tension in the membrane will also tend to lower the sensitivity of the microphone.
Prior art implantable microphones for use with hearing systems have comprised an electret microphone disposed within an air cavity, enclosed by a stretched stainless steel membrane. The air cavity is hermetically sealed, necessitated by implantation in the body. The membrane is stretched tight and laser welded; the resulting system frequency response therefore has a low sensitivity and a sharp high frequency resonance peak. An improved device response would have high sensitivity, comparable to an electret microphone alone in air, and would be generally flat across the audio frequency, especially in the range of speech (500-4,000 Hz). Additional requirements for an improved implanted microphone include low distortion and low noise characteristics.
Traditional, non-implantable type microphones have an air cavity behind the membrane that is not sealed, with reference to the nearest surface behind the membrane. Traditional microphones are concerned with optimal membrane displacement, and typically have several air cavities which are used to influence the shape of the microphone response. An implantable microphone design that incorporates a membrane, enclosing a sealed chamber containing an electret microphone, is necessarily concerned with an optimal pressure build-up in the sealed cavity. This pressure build-up in turn displaces the membrane of the electret microphone. However, a sealed air cavity presents new challenges to the design and optimization of implantable microphones.
With the advent of fully implantable devices for stimulating hearing, there is a great need for implantable microphones that provide excellent audio performance. The present invention provides improved audio performance through improvement of microphone design.
The present invention provides implantable microphone devices that may be utilized in hearing systems, particularly in systems having bone mounted and other implantable drivers. The device comprises a flexible membrane disposed over a sealed cavity. The membrane may be made substantially flexible by etching or forming the membrane until it is very thin. Also, the sealed cavity may be limited to a very small volume which decreases the sealed air cavity acoustic compliance. Both of these examples simultaneously increase overall sensitivity of the device and move the damped resonance peak to higher frequencies.
In a preferred aspect an implantable microphone device is provided which comprises a housing and a membrane disposed over a surface of the housing to define a primary air cavity therebetween. A microphone assembly is secured within the housing. The microphone assembly has a secondary air cavity and an aperture which couples the secondary air cavity to the primary air cavity so that vibrations of the membrane are transmitted through the primary air cavity and aperture to the secondary air cavity. A microphone transducer is disposed in the secondary air cavity to detect said transmitted vibrations. Preferably, the microphone transducer comprises an electret membrane, a backplate, and electrical leads. Advantageously, a protective cover over the membrane is provided to protect the membrane from direct impact, where the protective cover is perforated to allow for free flow of vibration to the membrane.
In one configuration, the housing further includes a rear chamber. The rear chamber encases electric leads to the microphone, and provides external access to the leads through a hermetic feedthrough.
In yet another configuration, the membrane may comprise at least one compliance ring. Preferably, a plurality of compliance rings may be used. The compliance ring may be either etched or formed into the membrane or otherwise secured to it by any suitable means.
In a second aspect of the implantable microphone device, surface details are positioned on a surface of the housing. Preferably, the surface details may include pits, grooves, or at least one hole which connects the primary air cavity to a rear chamber of the housing. The surface details are provided to increase resonance peak damping.
In a third aspect, the implantable microphone comprises a housing comprising a rear chamber and includes a thin-walled tube section or other port opening for filling or evacuating specialty gases from said chamber. Filling the various cavities of the microphone with specialty gases decreases the acoustic compliance of those cavities. Accordingly, the housing further comprises a microphone assembly which may be vented, such that the gases can permeate each cavity of the implantable microphone. Alternatively, surfaces details on the housing, such as holes, may also connect the various cavities of the microphone device.
In a fourth aspect, the implantable microphone device, comprises a biocompatible material positioned proximate to the membrane. Preferably, the biocompatible material is biodegradable and degrades over time. Example materials include lactide and glycolide polymers. The position of the biocompatible material may vary from, for example, simple contact with only the front surface of the membrane to complete encapsulation of the entire microphone. This material provides protection from initial tissue growth on the microphone which may occur after implantation of the device. A volume occupying layer may be used to occupy a space between the membrane and an opposing surface of the biocompatible material. The volume occupying layer may naturally, over time, permanently fill up with body fluids or may comprise a permanent, biocompatible fluid-filled sack. In either form, these fluids will maintain an interface between the membrane and the surrounding tissue.
In a fifth aspect, the implantable microphone device comprises a microphone assembly with the secondary air cavity removed such that the electret membrane is directly exposed to the primary air cavity. The removal of the secondary air cavity creates a further reduction in overall air cavity volume which leads to a reduction in the acoustic compliance of the microphone.
In a sixth aspect, the implantable microphone device has a modified microphone assembly which eliminates the electret membrane. The assembly comprises an insulation layer secured on the inside surface of the implantable microphone membrane. An electret membrane-type material is, in turn, secured on the insulation layer. A backplate is disposed within the primary air cavity proximate to the insulation/membrane-type material combination. This aspect of the invention provides the advantage of a direct electret displacement, a lower overall component count, and an overall thinner microphone profile.